In the drilling and production of oil and gas wells, it is often desired to obtain information concerning conditions within a borehole. For example, tools and other objects may become lodged in the borehole during the drilling of a well. Such objects must be retrieved before drilling can continue.
In the operation and/or periodic maintenance of exploration, production or injection wells, it is frequently desired to obtain information about the construction and/or operating conditions of equipment located downhole. For example, detection of the onset of corrosion damage to well tubing or casing within a borehole enables the application of anti-corrosive treatments to the well. Early treatment of corrosive well conditions prevents the highly expensive and dangerous replacement of corrosion damaged well production components.
Other maintenance operations in a production well environment, such as replacement of various flow control valves or the inspection of the location and condition of casing perforations, make it highly desirable for an operator located at the surface to obtain accurate, real-time information about downhole conditions.
In fact, new regulations require operators of oilfields to perform a visual inspection of their safety/barrier valves after certain operations to verify cleanness to secure a further safe operation. This refers especially to BlowOut Preventers (BOP) which are large, specialized valves or similar mechanical devices, usually installed redundantly in stacks, used to seal, control and monitor oil and gas wells, and intended to prevent tubing (e.g. drill pipe and well casing), tools and drilling fluid from being blown out of the wellbore—or bore hole—, when a blowout threatens.
Various techniques have been proposed for obtaining information about the conditions within a borehole, well, pipe or other tubular constructions filled with contaminated fluid with an image sensor/camera. One example is disclosed in U.S. Pat. No. 4,938,060, to Halliburton (ex-OTIS), inv Sizer et al. It includes a method of bringing coiled tubing having an inspection sensor down into a wellbore to a selected location, injecting an optically transparent or acoustically homogenous fluid into the wellbore through the coiled tubing to form a slug of such fluid around the sensor, and transmitting signals from the sensor representative of well conditions to the surface. The method may be practiced to inspect only the region around the sensor at a selected depth in the well, or it may be continuously practiced to examine the length of the wellbore by producing the well and retrieving the coiled tubing and sensor at a controlled rate synchronized with the rate of well production.
One problem with U.S. Pat. No. 4,938,060 is that it is a slow and expensive solution, requiring the use of coiled tubing and the circulation and consumption of a large amount of transparent liquid. For instance, the coiled tubing may get stuck in e.g.
irregularities of the wellbore. Moreover, the inspection sensor may get damaged in its way down to the selected location.